Why does pulling out main speakers from wall improve sound?

Simple answer: Because they are designed to work that way. Mostly due to the increased use of subwoofers the speakers no longer need the bass reinforcement of a near wall or near corner set-up. 

The port in the back is for bass. The port should be away, no doubt, so that the bass does not become boomy. I also use a speaker that does not have a pass port in the back. But you have to realize that the speakers do have tweeters and mid-range sounds coming from the drivers. They first hit the side walls and the wall behind you and then reach the front wall - from where they are again reflected back to you. My belief is that the more time it takes for these reflections to bounce off the front wall (wall behind the speakers) and then come to you, gives you more perception of depth. By "treating" the front wall, you are killing off some of them. But you do not want to kill all of them and have a dead sound. Hence some folks have reflectors/diffusers instead of absorbers on the front wall.

This is in layman's terms. Someone more technical might be able to provide a better explanation.

Speaker placement is very critical, and can make a huge difference in your room. Get a roll of tape and start marking the floor. Get a base line for your room size and start. Do you have any room treatment in your room at all. That is a huge rabbit hole to go down. 

It’s all about reflections and cancelations. A pebble dropped in the center of the pool creates a uniform wave pattern. A pebble dropped close to the side creates cancelations and a confused wave pattern. Stand behind your speaker, can you still hear the music?

My issue is the bass sounds so much better when I stand up. I’m pretty sure it’s floor reflection but don’t know how to correct it. My subs are stacked so the top sub center of the driver is about 34" and both are pulled out from the wall about the same 34" to the front of the cabinet. It’s almost 30’ to the back wall so I don't think that's where the issue is.

The qualitied among us can give the technical/physics lesson.

It just does, so DO IT! Most speakers do best when given the breathing room.